Field of the Invention
This invention relates to electrically powered instruments for determining the angular orientation of a line or plane with respect to a portion of a suspended member, such as a plumb bob, having its position determined by gravity and providing an electrical output or signal, such as a visual and/or aural indication when the line or plane has achieved a desired predetermined angular orientation.
A well known instrument for indicating the angular orientation of an object with respect to the earth's center of gravity is the conventional builder's level. The existing levels are sufficiently accurate for either construction or scientific applications and generally comprise a rectangular elongated frame, often made of metal or wood, and containing one or more curved glass tubes or vials almost completely filled with alcohol, except for a small visually discernable air space or bubble. The glass tubes are usually arranged on the frame in such a way as to indicate either a vertical or horizontal position of the object against which the frame is abutted. The mid-portion of each glass tube is provided with a pair of spaced lines which can be etched or painted on the tube. If the object is in the desired position, such as horizontal or level, the air bubble comes to rest centered within the space between the two lines.
This type of level, commonly called a "spirit level", has remained virtually unchanged for many years. Its primary deficiency is that it is difficult to read. The glass tube is typically less than two (2") inches long and the bubble is typically less than 3/8ths of an inch wide. Poor light conditions (too bright or too dim) or poor eyesight may make it difficult or impossible to read. Even if the user has good eyesight, it is still often difficult to determine when the clear bubble, in an almost clear liquid, has come to rest perfectly centered in the space between two fine lines.
Generally, the spirit level cannot be read at all at a distance further away than about eighteen inches (18"), even with good eyes under good light conditions. Yet, the spirit level must often be used in dimly lighted places where the eyes cannot easily follow it (for example, an overhead beam or a joist below the floor level). A carpenter will usually place his level at the center of a long beam (for accuracy), but then he will have to adjust the position of the beam from its end, many feet away from the level. To overcome in part the foregoing disadvantage, some of the spirit levels are provided with a small light source to illuminate the tube. However, even a flashlight will not make it readable from a distance.
Some levels, presently available on the market, have been provided with light or sound devices which indicate visually or aurally when a true horizontal or true vertical position is achieved. For example, U.S. Pat. No. 3,233,235 describes a signaling level which uses a pendulum provided with a magnet pointer for closing an electric circuit to provide a signal that a true vertical or true horizontal has been indicated.
Another form of a level using a pendulum is described in U.S. Pat. No. 4,094,073, wherein a weighted pendulum achieves a contact in an electrical circuit to give a signal for a true horizontal or vertical positioning. In another form of a level, U.S. Pat. No. 4,094,073, there is described an angle detector using a weighted pendulum.
In all of the foregoing patents, the weighted pendulum is pivotally supported on a pin. This type of pendulum support possesses an inherent amount of friction which tends to interfere with achieving of a true horizontal or true vertical positioning.